Polarisation of electromagnetic waves

Introduction to Polarisation

Welcome! In this chapter, we are exploring a fascinating property of waves called polarisation. This concept is a vital part of the "Physics in Action: Communication" module because it explains how we can control electromagnetic waves to carry information more effectively, from satellite TV to your mobile phone signals.

Don't worry if this seems a bit abstract at first. We will use simple analogies to help you visualize what is happening to the waves at a microscopic level.

What is Polarisation?

To understand polarisation, we first need to remember that electromagnetic waves (like light, microwaves, and radio waves) are transverse waves. This means the oscillations (vibrations) are at right angles (\(90^{\circ}\)) to the direction in which the wave is travelling.

Most light sources, like a lightbulb or the Sun, are unpolarised. This means the waves are vibrating in every possible direction perpendicular to the path of travel—up and down, left to right, and every diagonal in between.

Polarisation is the process of restricting these vibrations so that they occur in only one single plane. Once this happens, we say the wave is plane-polarised.

Note: Only transverse waves can be polarised. Longitudinal waves (like sound) cannot be polarised because their vibrations are always parallel to the direction of travel.

The "Picket Fence" Analogy

Imagine you are holding a long rope passed through the gaps in a vertical picket fence.
1. If you shake the rope up and down (vertically), the waves pass through the fence easily.
2. If you shake the rope side-to-side (horizontally), the waves hit the bars of the fence and are stopped.
The fence acts as a polariser, only allowing waves vibrating in a specific "plane" to pass through.

Key Takeaway: A polariser only allows waves vibrating in one specific direction to pass; it absorbs or reflects vibrations in other directions.

Evidence of Polarisation: Light

How do we know polarisation actually happens? We can use materials called polarising filters (often called Polaroids) to provide evidence.

1. Using a single filter

If you look at unpolarised light through one filter, the light appears slightly dimmer because roughly half of the wave energy (the vibrations not aligned with the filter) is blocked.

2. Using two filters (The "Crossed Polaroids" Experiment)

This is the classic evidence used in physics labs:
- Take two polarising filters and place them one in front of the other.
- If the "transmission axes" of both filters are parallel (both vertical), light passes through both.
- If you slowly rotate one filter, the light gets dimmer.
- When the second filter is at \(90^{\circ}\) to the first, the light is completely blocked. This is because the first filter polarised the light vertically, and the second filter (now horizontal) blocks all vertical vibrations.

Evidence of Polarisation: Microwaves

In your practical work, you will likely use a microwave transmitter and receiver to demonstrate polarisation. Microwaves are particularly useful because the transmitter usually produces waves that are already polarised.

The Metal Grille Practical

You can use a grille made of parallel metal rods to test the polarisation of microwaves:
- Point the transmitter at the receiver.
- Place the metal grille between them.
- Surprising Result: When the metal rods are vertical (aligned with the electric field of the microwaves), the signal at the receiver drops to nearly zero. The metal rods conduct electricity, allowing the electrons in the metal to vibrate with the wave, absorbing its energy.
- When the rods are horizontal (perpendicular to the vibrations), the microwaves pass through to the receiver.

Quick Review:
- Light filters: Pass light when aligned with the plane of vibration.
- Metal grilles (for microwaves): Absorb waves when the rods are parallel to the plane of vibration.

Polarisation in Communication

Polarisation isn't just a lab trick; it is essential for modern communication technology. Because we have limited space in the "radio spectrum," we need to be clever about how we send signals.

1. Avoiding Interference:
Broadcasters can send two different TV signals on the same frequency without them interfering with each other. They do this by polarising one signal horizontally and the other vertically. Your TV antenna is then aligned specifically to pick up only the plane of vibration intended for your local transmitter.

2. Satellite Communication:
Satellites use different polarisations to double the amount of data they can send back to Earth using the same range of frequencies.

3. Reducing Noise:
In communication, "noise" is any unwanted signal. By using polarised receivers, engineers can filter out unpolarised background noise, leaving a clearer, stronger signal.

Common Mistakes to Avoid

1. Thinking sound can be polarised:
Examiners love to ask this! Remember: Sound is longitudinal. Polarisation is only for transverse waves like electromagnetic waves.

2. Misunderstanding the metal grille:
Many students think the microwaves "slip through" the gaps between the bars like the rope analogy. Actually, for microwaves, the rods absorb the energy when they are parallel to the vibrations. If the rods are parallel to the wave's electric field, the signal is blocked.

Key Terms Summary

Polarisation: The process where the oscillations of a transverse wave are limited to a single plane.

Unpolarised: A wave vibrating in multiple planes perpendicular to the direction of energy transfer.

Plane-polarised: A wave that vibrates in only one plane.

Transmission Axis: The direction in which a polarising filter allows oscillations to pass.

Next Step: You might want to briefly review Digital images, sampling and signalling to see how these polarised signals are converted into the data we use every day.